1. Field of the Invention
The present invention relates to a charged particle beam system.
2. Description of Related Art
In a charged particle beam system such as a transmission electron microscope (TEM) or a scanning electron microscope (SEM), a sample is required to be placed in position with high positional accuracy. The positional accuracy referred to herein is a restraining capability of maintaining a sample at rest after an operation for placing the sample in position is complete. If the sample keeps moving unintentionally after the positioning operation is complete, the sample is said to drift.
For example, JP-A-2010-157491 discloses an object positioning device for use with a charged particle beam system such as a transmission electron microscope. In this disclosed JP-A-2010-157491, when a sample is introduced into the electron optical column of the microscope or when the sample stage is moved a great distance to search for a desired field of view for observation, the sample drive unit deforms elastically. As a result, a large stress is introduced as a spring component. When this stress is relieved by the action of a damper component also possessed by the drive unit, the drive unit itself moves, causing the sample to drift. This deteriorates the restraining capability.
FIG. 10 shows a model M for illustrating the drift. If this model M is compressed by applying a certain force across the model M, a damper D1 cannot follow instantaneous movements. Therefore, only a spring S1 contracts. The damper D1 is then pushed in with the elapse of time and the spring S1 begins to be restored to its original shape. When the damper D1 stretches to some extent, the stretching force of the spring is lost, and the progress of the stretch comes to a stop.
If the model M is applied to the sample stage of the transmission electron microscope, then a deformation of an O-ring used for vacuum sealing purposes, a deformation of a resinous part used for electrical insulation, and grease applied to various contact surfaces correspond to damper components. An elastic deformation of the drive unit corresponds to a spring component. The O-ring acts as a damper component and also as a spring component because the rubber that expands and contracts causes an elastic deformation.
FIG. 11 shows one example of damper component. Cross sections of O-rings are schematically shown in FIG. 11. Damper components are now described while taking O-rings as examples.
The drive unit operates while producing a magnitude of elastic deformation corresponding to frictional force produced during the operation. After the drive unit has come to a halt, the elastic deformation remains as a spring component (stress). After the operation of the drive unit has stopped, the stress in the drive unit is relieved with time because of the damper effect of the O-ring. At this time, the damper D1 shown in FIG. 10 is pushed in, resulting in a variation in the shape of the O-ring. According to the magnitude of the stress introduced during operation, the shape of the cross section of the O-ring varies in the direction indicated by the arrows in FIG. 11. As a result, the sample drifts, i.e., the sample moves. When the stress in the drive unit disappears or when the stress in the drive unit is balanced by another stress counteracting the former stress, the drift comes to a stop.
A transmission electron microscope has high resolution. Subjects to be observed have sizes on the order of nanometers. In recent years, making observations on an atomic scale using a transmission electron microscope has been increasingly carried out. Therefore, the amount of drift that the sample is allowed to exhibit during imaging is on a subatomic level. In order to prevent deterioration of the spatial resolution, the exposure time is restricted by the speed at which the sample drifts and by the minimum pixel size of the detector. If the drift is small, the shooting time can be prolonged and so the S/N can be improved while retaining the resolution of the taken image.
Furthermore, in transmission electron microscopy, energy-dispersive X-ray spectrometry for performing elemental analysis using information about X-rays produced from a sample or electron energy loss spectroscopy for analyzing elements constituting a substance or its electronic structure from energies lost from impinging electrons by a sample is often performed as well as imaging. For such spectrometry or spectroscopy, data need to be collected for a much longer time than where an image is taken. If the sample drifts during data acquisition, the spatial resolution will be directly impaired. Therefore, if the sample can be kept at rest without drifting after the sample is placed in position, then high spatial resolution and high S/N can be accomplished.
One conceivable method of preventing drifting is to cancel drift by movement of the sample. In particular, the sample is once brought onto a target field of view. Then, the sample is intentionally made to pass over the target field of view. The sample is then returned onto the target field of view. That is, a negative amount of drift is introduced, thereby canceling the true drift.
The drift speed and the stress in the drive unit causing the drift decrease with time but do not vary linearly with time. The degrees of variations change depending on the magnitude of the stress introduced into the drive unit and on the damper component. The magnitude of the stress introduced into the drive unit is affected by the damper component such as grease and depends on the amount of motion made until the sample reaches the target field of view, on the moving speed, and on the hysteresis.
FIG. 12 is a graph in which drift speed is plotted against elapsed time for various amounts of motion (100 μm, 10 μm, 5 μm, and 1 μm) made prior to positioning. The drift speed decreases exponentially with time. Furthermore, the amount of drift decreases with reducing the amount of motion made prior to positioning. As can be seen from FIG. 12, when the amount of motion is different, the curve of the graph is different in tilt. Therefore, it is difficult to cancel the drift by introducing a negative amount of drift.
Furthermore, the frictional forces occurring on individual sample stages are not uniform. The stress varies depending on the frictional force. In addition, individual dampers vary among each other. Therefore, behaviors of drift vary in a complex manner. Especially, an O-ring has both a spring component and a damper component and further complicates the phenomenon.
Accordingly, it is very difficult to intentionally introduce a negative amount of drift so as to cancel out the true drift as described previously.